The invention relates to communication networks in general and more particularly to a protocol for stripping data from a network using early token release.
The use of ring communication networks for transferring data is well known in the prior art. These networks consist of a set of stations serially connected to a communication medium. Information is transferred sequentially, bit by bit, from one station to the next. The inactive stations are bypassed while the active stations regenerate and repeat each bit. Furthermore, stations are used as the means for attaching one or more devices (terminals, workstations, computers, displays, etc.) which communicate with other devices on the network.
An originating station transfers its information message onto the ring. The message circulates, passes one or more intermediate stations, until it arrives at the destination station where it is copied. In some implementations, the destination station strips the message from the ring. In other implementations, the message remains on the ring until it arrives at the originating station where it is stripped from the ring. Usually, the ring also includes a monitor station which is responsible for stripping messages that have not been stripped by the station normally responsible to perform the stripping. In addition, the monitor station may perform basic housekeeping functions for the ring.
An access method protocol is the mechanism which enables a station to transmit on the ring. One such access protocol is referred to as the token insert access protocol. This protocol provides a station with an opportunity to transmit whenever it receives a free token. The free token is usually generated by an upstream station. The token access protocol allows only one message to be on the ring at any particular instant of time. One characteristic of a token ring network is ring latency, which is the time, measured in bit times at the data transmission rate, that is required for a signal to propagate once around the ring. With a token access protocol, when the messages are relatively short, compared to the latency of the ring, only a fraction of the available bandwidth is used. Failure to use all of the available bandwidth reduces the efficiency of the ring because a series of idle patterns must be transmitted to fill up the vacancy on the ring.
In a ring which has a latency that greatly exceeds the duration of the shortest message and which employs an access protocol that allows multiple messages to co-exist on the ring, it is desirable for each station that has originated a message to strip this message, after the message has circulated once around the ring, such that the message will not be accepted a second time by a destination station.
A station must be able to distinguish between messages that require stripping actions by this station and those messages which require stripping actions by another station. In order to make this distinction, some serial bufferinq of a message may be required. It is desirable to keep such buffering to a minimum in order to reduce costs and to reduce added ring latency.
The prior art uses different types of stripping protocols to dispose of exhausted messages. One stripping protocol requires the target station to strip each message that it receives. This approach recovers bandwidth promptly, however, it is unsuitable for broadcast messages, destined for multiple target nodes. This approach also does not provide feedback to the original sending station.
Another stripping protocol requires that the sending station strip each message which it has sent, after this message comes back around the ring. This approach is suitable when the access protocol is such that no messages originated by another station can arrive at the sending station prior to the return of any messages which this sending station has originated.
U.S. Pat. No. 4,567,590 issued to Bederman discloses a message stripping protocol for a ring communication network in which a plurality of stations are used to strip expendable messages from a serial ring. One station uses either the destination field or the source address field as the basis for removing a portion of the message. Another station uses the status of a control indicia as the basis for removing the remaining portion of the message from the ring. By distributing the stripping between multiple stations on the ring, a significant reduction in ring latency is achieved.
Under normal token release a station holds the token until it has recognized the source address in the header of the returning frame. If the station completes frame transmission before receiving the header of the returning frame, the station must transmit idle characters until the header is received. If the transmitted frame is shorter than the bit capacity of the ring, bandwidth is wasted by transmitted idle characters. With the development of token rings that can handle data rates of 16 Mbps or greater, the number of bytes required to fill the ring increases and more idle characters are needed to fill the ring after frame transmission, adversely affecting efficiency. The concept of early token release allows a transmitting station to release the token as soon as it completes frame transmission, regardless of whether the frame header has returned to that station. With the early release of the token, the token is available to other stations sooner than in normal token release, and no idle characters fill the ring between the end of the transmitted message (consisting of the frame and an inter-frame gap) and the token. Thus, early token release allows multiple frames to be on the ring at the same time. A workstation on the ring now can transmit a token immediately after sending a frame of data, rather than waiting for its frame to return. Early token release improves the efficiency of the ring when the transmitted frames are shorter than the bit capacity of the ring.
In a token ring using the early token release scheme, the basic protocol has two problems. First, a transmit station will strip another station's frame if its own frame is corrupted. Second, a monitor station cannot detect an error condition in which all of the frames on the ring are being stripped by the wrong stations. This invention describes a scheme which will solve both of these problems.